Take a moment to think about the answer to this question: Where in the United States would you expect an area to feel about 3,000 earthquakes in a 16-month period, some citizens request earthquake insurance to protect their property and wonder if heavy precipitation plays a role in these seismic events?

The location is not in California or Alaska, as you may have thought, but in Carroll Township, northern York County. Beginning on Sunday, October 5, 2008, at about 6:30 a.m., the area felt a 2.0 tremor that gave residents a rude awakening so early in the morning. The earthquake was recorded on the Columbia University’s Lamont-Doherty Earth Observatory (LDEO) seismic network. The closest station belonging to this network is Millersville University, which is available to view in real time online.

I reported to the area the following afternoon using the data from the LDEO which established the epicenter. Numerous interviews with residents, police reports written by the Carroll Township Police Department, surveys in local newspapers to returned to me, citizens directly contacting me and telephone interviews allowed a isoseismal map showing a more defined epicenter location was quickly established. Although only using responses from area residents and no scientific data included, this was a good starting point.

At first, knowing that in the immediate area of the epicenter were a dozen 19th century iron mines, mostly now water-filled, I viewed a mine roof collapse as a possible cause. As several days passed and more tremors reported, that hypothesis quickly went out the door. I contacted my friend Dr. Charles Scharnberger, professor emeritus of geology at Millersville University, to help in the investigation. Dr. Scharnberger is one of the seismic experts on the East Coast and maintains the seismograph at the university.

A photograph of one of the IRIS seismographs used in the summer of 2009.(Photo: Submitted)

On Sunday, October 19, 2008, about a dozen tremors were reported. It seemed like “Mother Nature” was throwing the earthquakes into northern York County mostly on Sundays in the early stages. Dr. Scharnberger, Helen Delano of the Pennsylvania Topographic and Geologic Survey in Middletown and myself were invited to host a “town meeting” to allow us to present a brief discussion of what may be occurring and answer any questions from residents. With only several weeks into the events, we had very little information and certainly few answers to what was occurring. Through this meeting, my database informing people to contact me to report any shaking or booms grew. The more information gathered would be beneficial for us to put together theories.

York College of Pennsylvania Earth Science Professor Dr. William Kreiger and his son Jason also collected additional data from residents which was added to our information. Dr. Scharnberger contacted Columbia University inquiring if they could install their portable seismic network in the Dillsburg area to give us scientific data to accurately locate the tremors. Their network was active from late October into early December 2008 with the tremors continuing. The residents were beginning to accept these small booms as a part of life and concern over the possibility of a major earthquake decreased. Reports from our database of citizens continued to come in as tremors occurred. There was an increase in activity in April and October, 2009. One day in April, 2009, 54 tremors occurred. With no seismic network operating in the area at that time, the tremors were based on citizen reports.

An isoseismal map of the October 5, 2008 tremor. Inside of Zone III was the epicenter.(Photo: Submitted)

In June, 2009, another community meeting was held as a request by Carroll Township supervisors. We had a better handle on the events now compared to the earlier community meeting. The events were following the pattern of what we call an earthquake swarm. A swarm is a series of small tremors that doesn’t contain foreshocks, a main event or after-shocks. It was predicted that the tremors would stop one day as mysteriously as they started. With Columbia University’s report not complete at this time, a local resident who loves scientific research supplied monies to install another network to continue to gather data. With the efforts of Pennsylvania State University geology students and the IRIS seismic network based in New Mexico, the system was used for much of the summer, again recording tremors. By September, 2009 the LDEO report and the IRIS information were complete. A map showing numerous epicenters along Old York Road and Brandon Lane confirmed the isoseismal maps compiled earlier.

By November, 2009, our database of reported tremors had grown to over 800. Our seismograph networks, however, suggests that 3 times that number of tremors were recorded. In other words, the citizens were not feeling or hearing many of the tremors, probably due to the low magnitude of them. We hosted our final community meeting at Northern High School in November presenting the most up-to-date information. The swarm did terminate in the early spring of 2010. Over 1,000 reported tremors are locked into our database, but we can safely suggest that Carroll Township had 3,000 tremors during the swarm. Several booms have been reported by our faithful “reporters” since then. Many of those reports were booms from firing range practice taking place at Indiantown Gap, some 33 miles to the northeast of Dillsburg. However, as recently as November 12, 2017 (yes, on another Sunday), one of my faithful reporters emailed me some details that was a tremor, a very small one only felt by a household or so. Ironically, this tremor occurred one day after a 1.5 magnitude quake near Olney, Maryland.

This is Part 1 about the Dillsburg swarm. My next posting will cover the geology and theories attached to this subject. If you are interested, do an Internet search for “Dillsburg Earthquake Swarm” to find several articles or go to my website at jonesgeo.com.

Jeri Jones writes regularly about geology in York County.

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Jeri Jones, program coordinator for York County Parks, retired in August. While he has many memories of the parks, "certainly my real love is Christmas Magic."(Photo: Teresa Boeckel, York Daily Record)